1) Best way to avoid underwatering and overwatering situations for houseplants is to provide the sufficient amount of water that means only provide the required amount of water until the soil becomes wet not soggy.(avoid standing water in the pot for longer periods).
2) Avoid keeping the plant in direct sunlight or indicrect sunlight for longer period.
3) Here's what to do if your plant has Bacterial & Fungal leaf infection:
a) Isolate the infected plant.
b) Remove and destroy any infected plant material.
c) The plant can be treated to prevent further damage, using a copper fungicide. Hot tip: Avoid splashing water on the foliage since this spreads the fungi.